This invention relates to a thermoplastic resin composition comprising a polyphenylene ether (hereinafter referred to as "PPE"), a saturated polyester, an organic phosphorous acid compound and, if necessary, an acid acceptor and having excellent mechanical strength, and a process for preparing the same. The above composition is an engineering plastics useful as a connector, an ignition manifold, a coil-sealing material, a gear and a crank shaft and also as an industrial material such as parts for automobiles and parts for electric equipments having excellent solvent resistance and heat resistance.
PPE has been used as an engineering plastics having excellent heat resistance, dimensional stability, nonhygroscopicity and electric characteristics. However, it has drawbacks that melt fluidity (flowability when melting) is bad to make molding processing such as injection molding and extrusion molding difficult, and also a molded product thereof has poor solvent resistance and impact resistance.
On the other hand, a saturated polyester represented by polybutylene terephthalate has been used widely as an engineering plastics having excellent molding processability, solvent resistance and mechanical strength in the fields of parts for automobiles and parts for electric and electronic equipments. However, a molded product of the above resin has drawbacks that molding shrinkage rate and linear expansion coefficient are high, and also rigidity is greatly lowered at high temperature.
If a composition having both good properties of PPE and the saturated polyester and compensating undesirable properties each other can be obtained, an excellent resin material having a wide application field can be provided, and it can be said that industrial significance of such a material is extremely great.
Compositions in which both resins are simply melted and mixed have been described in, for example, Japanese Patent Publication No. 21664/1976 and Japanese Provisional Patent Publications No. 75662/1974 and No. 159847/1984. However, in such a simple blend system, PPE and the saturated polyester have poor compatibility with each other so that a composition can hardly have a uniform and fine mixed state. As a result, even when such a composition is used, impact resistance, heat resistance, dimensional precision and solvent resistance of a molded product thereof cannot be improved satisfactorily, and appearance of a molded product is worsened.
It has been generally known that the compatibility can be improved by adding a block and/or graft copolymer comprising PPE and the saturated polyester to a composition. The block and/or graft copolymer can be prepared by PPE and a saturated polyester both having polyfunctional groups which react with and bond to each other. It has been known that as the polyfunctional group, PPE has a phenolic hydroxyl group represented by the formula (a): ##STR2## wherein R.sup.1 and R.sup.1 ' each represent a halogen atom, a primary or secondary alkyl group, an aryl group, an aminoalkyl group, a halohydrocarbon group, a hydrocarbonoxy group or halohydrocarbonoxy group; and R.sup.2 and R.sup.2 ' each represent a hydrogen atom, a halogen atom, a primary or secondary alkyl group, an aryl group, a halohydrocarbon group, a hydrocarbonoxy group or a halohydrocarbonoxy group, or the formula (b): ##STR3## wherein R.sup.1, R.sup.1 ', R.sup.2 and R.sup.2 ' have the same meanings as defined above, at a molecular end thereof.
On the other hand, it has been known that the saturated polyester has an ester structure as a matter of course or a carboxylate group at a molecular end thereof.
In general, bonding reaction of the above phenolic hydroxyl group and carboxylic acid, i.e. direct esterification is not easily carried out so that various aids for activating the reaction and other means have been developed.
There has been conventionally known a method of using triphenyl phosphite as an aid for direct esterification of a phenolic hydroxyl group and carboxylic acid (Yamazaki and Higashi, "Advances in Polymer Science", 38, 1 (1981)). It is considered that a block copolymer of PPE and a saturated polyester can be obtained by applying the above reaction to a phenolic hydroxyl group at the end of a molecular chain of PPE and a carboxyl group at the end of a molecular chain of a saturated polyester.
In U.S. Pat. No. 4,672,086, it has been described that 5 to 15 % by weight of a phosphor compound (including a phosphorous acid compound) is added to a composition of PPE and a polyester for the purpose of imparting flame retardancy. However, even when tris(nonylphenyl) phosphite described therein is used, a composition having improved compatibility and good physical properties cannot be obtained.
In PCT Provisional Patent Publication No. WO 91/08250 (which corresponds to U.S. Pat. No. 5,124,411), there has been disclosed a method for preparing a graft or block copolymer comprising PPE and a polyester, using an extremely common phosphorous acid compound having a P(OR).sub.3 structure such as triphenyl phosphite. In the above method, it is an essential requirement that a phosphorous acid compound decomposed by reaction or side reaction and an unreacted phosphorous acid compound are removed in order to accelerate reaction. However, physical properties such as impact resistance of the resulting composition are not satisfactory. It is considered that this is because reaction activity as an ester condensation agent is not sufficient so that an amount of a formed block and/or graft copolymer is small and compatibility is not improved sufficiently.